1. Field of the Invention
The present invention relates to scanning electron microscopes and similar instruments that employ scanning electron beams.
2. State of the Art
It is well known to use scanning electron microscopes for measurement and inspection purposes in, for example, the semiconductor manufacturing industry. Scanning electron microscopes, as a result of the short wavelengths of their source electrons, have several advantages over optical microscopes. For example, scanning electron microscopes can achieve resolutions from about 20 to 200 Angstroms, but the limiting resolution of optical microscopes is about 2,500 angstroms (i.e., 0.25 microns). Further, scanning electron microscopes provide depths of field several orders of magnitude greater than optical microscopes.
At the high magnifications that are typical of scanning electron microscopes, image quality can be severely impaired by even slight vibrations. That is, vibrations of a stage with respect to the electron beam in a scanning electron microscope appear in the images produced by the microscope. Although structural (low frequency) vibrations can usually be attenuated by mounting scanning electron microscopes usually on elastic vibration-isolating structures, such structures are not necessarily effective in attenuating acoustic and other high-frequency vibrations.
As is disclosed in the above-identified co-pending application, systems can be provided that use velocity transducers, such as seismometers, to measure the velocity of vibrations in two or more orthogonal directions. In operation of the systems, the velocity signals are integrated to produce signals whose magnitudes are proportional to displacements, and those signals can be used to alter the scanning pattern of an electron beam in such a way as to minimize, or eliminate, the appearance of vibrations in images produced by the scanning electron microscope.
However, as is further explained in the co-pending application, when the signal amplitude gain of a velocity transducer is adjusted such that its output signals vary linearly with the amplitude of sensed vibrations, the phase of the output signals may either lead or lag the vibrations. Moreover, the phase response may vary with the frequency of sensed vibrations.